As for the method for evaluating noise (photon noise) of an X-ray CT apparatus, SD in an image has been used. The SD in an image indicates dispersion degree from the average value of pixel values, and the higher the noise is the greater the SD value becomes. Particularly, in the case of observing (diagnosing) a region in the object wherein the difference in CT values are small, i.e. CT values are comparatively homogeneous, it is necessary to reduce noise, i.e. to suppress SD value of the image.
One of the methods for reducing noise is to increase the number of photons (volume of information). It is known that the number of photons to be irradiated per unit time increases approximately in proportion to the square of tube voltage. Given this factor, tube voltage is increased in order to reduce noise, but since the number of photons increase by doing so, radiation exposure to the object also increases. Therefore, it is important to reduce radiation exposure to the object as much as possible, and perform scanning of the object with minimum radiation exposure necessary for diagnosis.
Also, attenuation ratio of X-rays at the time of transmitting to the object (ratio of X-ray irradiation with respect to the transmitted X-ray) is determined by the product of X-ray attenuation coefficient and transmitting distance. Therefore, the greater the X-ray attenuation coefficient of the object is and the longer the transmitting distance is due to the size of the object the greater the attenuation ratio becomes, which leads to the reduction of the number of photons and increase of noise.
Since the shape of cross-sections of an object is close to an ellipse, in the case of scanning the object using an X-ray CT apparatus by setting a constant tube current without depending on a revolving position (view angle), the number of photons after transmitting through the object varies by each view angle, thus the amount of noise also varies for each view angle. Since a CT image is acquired by reconstructing projection data at various view angles, image quality of the CT image is strongly influenced by projection data at the view angle having the most noise. In other words, even projection data of a certain angle has a small amount of noise, if the projection data of another angle has much noise, the CT image will have much noise, i.e. a high image SD value. For this reason, the technique for stabilizing noise for each revolving position by modulating tube current during revolution has been used in recent years.
In JP-A-2001-276040, the X-ray CT apparatus is disclosed which generates an elliptic model by presuming the shape of cross-sections of an object as an ellipse based on the projection value profile acquired by scanogram projection and controls the X-ray dose to be irradiated to the object with respect to every view angle using the generated model. In accordance with this X-ray CT apparatus, since the number of photons after being transmitted through the object can be stabilized regardless of the view angle if the generated elliptic model is adequate, it is possible to acquire CT images having the target image SD value while suppressing unnecessary radiation exposure.